1. Field of the Invention
The present invention relates generally to a DC (direct current) restoration circuit, and more particularly, to a DC restoration circuit for restoring and compensating a low frequency component of a digital signal if the low frequency component is lost in a digital signal transmission, recording or reproducing system such as a digital video tape recorder (referred to as digital VTR hereinafter).
2. Description of the Background Art
Conventionally, a digital signal such as a non-return-to-zero (referred to as NRZ hereinafter) signal comprises a low frequency region including a DC component. FIG. 1 is a diagram showing frequency characteristics of such a digital signal, where the abscissa represents a relative frequency and the ordinate represents a relative level. In FIG. 1, a solid line represents frequency characteristics of the above described digital signal comprising low frequency component. In transmitting the digital signal, a signal component in the low frequency region, which is represented by a broken line may be cut off.
More specifically, for signal transmission through, for example, a transmission path, the transmission of a digital signal and the supply of power may be simultaneously made using a pair of signal lines. In such a case, a low frequency component in the digital signal may be cut off to transmit only a signal having a frequency component which is higher than a given value, and a low frequency region including a DC region may be allocated to the supply of power.
Additionally, for recording and reproducing of the digital signal in, for example, a digital VTR, the low frequency component in the digital signal is lost due to differential response characteristics of a magnetic head.
As described in the foregoing, when the low frequency component in the digital signal is cut off, the base line of the digital signal wanders. FIG. 2 is a waveform diagram showing the digital signal having its base line wandering, where a solid line represents a waveform of the digital signal and a dot and dash line represents the base line thereof. If and when the base line wanders as shown in FIG. 2, it becomes difficult to precisely determine whether the digital signal is at a high level or a low level on a receiving or reproducing side of the digital signal. As a result, there is an increased possibility that a code error occurs.
Therefore, conventionally, on the receiving or reproducing side of the digital signal, a DC restoration circuit, using a so-called quantized feedback method, is utilized in order to compensate for the low frequency component which is lost in the digital signal. Such a DC restoration circuit is disclosed in, for example, Japanese Patent Laying-Open Gazette No. 129975/1985.
FIG. 3 is a schematic block diagram showing one example of such a conventional DC restoration circuit, and FIG. 4 is a diagram showing frequency characteristics of the circuit shown in FIG. 3. In FIG. 4, the abscissa represents a relative frequency and the ordinate represents a relative level, as in FIG. 1.
In FIG. 3, a digital signal having a frequency component represented by a solid line in FIG. 4 is input to an input terminal 1 from a predetermined transmission or reproducing system. This digital signal corresponds to a signal having its low frequency region represented by the broken line in FIG. 1 out of the inherent frequency component represented by the solid line shown in FIG. 1 cut off by the above described various causes. This digital signal is applied to one input terminal 3 of an adder 2. In addition, an output terminal 4 of the adder 2 is connected to an input of a determination circuit 5. This determination circuit 5 performs processing such as retiming and data regeneration with respect to an input signal. A data signal "1" or "0" is outputted from an output terminal 8. A low frequency component extracted by a low-pass filter (LPF) 6 in this data signal is applied to another input terminal 7 of the adder 2 as a feedback signal. It is assumed that pass characteristics of the LPF 6 are set to be approximately equal to low frequency cut-off characteristics of the digital signal applied to the input terminal 1. As a result, a low frequency component as represented by a broken line in FIG. 4 is extracted from the LPF 6 and added to the input signal by the adder 2, so that the lost low frequency component in the input digital signal is restored.
Meanwhile, in the conventional DC restoration circuit as shown in FIG. 3, the feedback value from the LPF 6 is always constant. Thus, in the case of recording and reproducing of the digital signal in, for example, the digital VTR, if the amplitude of the input digital signal is wanders due to spacing loss, drop out or the like, a balance between a compensation signal from the LPF 6 and a signal to be compensated for from the input terminal 1 is destroyed because the feedback value is constant as described above, so that stable compensation for a low frequency component can not be made.
FIG. 5 is a diagram showing one example of a DC restoration circuit proposed to solve such a problem, which is disclosed in, for example, Japanese Patent Laying-Open Gazette No. 123064/1986. The DC restoration circuit shown in FIG. 5 is the same as the DC restoration circuit shown in FIG. 3 except for the following. More specifically, a variable gain amplifier 9 is provided between an LPF 6 and an input terminal 7 of an adder 2. In addition, there is provided a level detector 10 for detecting a level of a signal input to an input terminal 1. More specifically, this level detector 10 has a function of rectifying and further smoothing the input signal. The gain of the variable gain amplifier 9, i.e., the feedback value from the LPF 6 is controlled by a control signal from the level detector 10. More specifically, in the DC restoration circuit shown in FIG. 5, the feedback value is controlled according to the level of the input digital signal, so that stable compensation for a low frequency component can be made.
However, the level detector 10 in the DC restoration circuit shown in FIG. 5 also detects a level of a low frequency component including a DC component, in the input digital signal. Thus, considering a case in which the base line of the input digital signal wanders by an extremely large amount, even if the amplitude itself of the input digital signal does not significantly wander, the wandering of the base line appears at an output of the level detector 10.
FIGS. 6(a) and (b) are waveform diagrams for explaining the principle on which such a phenomenon occurs. More specifically, when a digital signal (represented by a solid line) having a base line (represented by a dot and dash line) which greatly wanders, as shown in FIG. 6(a), is input to the level detector 10, this signal is rectified with respect to a reference level represented by an arrow A in the figure, as represented by a solid line in FIG. 6(b) and then, smoothed. As a result, an output as represented by a broken line in FIG. 6(b) is obtained from the level detector 10. Thus, even if the amplitude of the input digital signal (solid line) itself is approximately constant as shown in FIG. 6(a), the output of the level detector 10 (represented by a broken line in FIG. 6(b)) wanders due to the wandering of the base line (dot and dash line). As a result, the gain of the variable gain amplifier 9 is changed. More specifically, the feedback value from the LPF 6 wanders almost independently of the level of the input signal itself due to the wandering of the base line of the input signal, so that a balance between a compensation signal from the LPF 6 and the signal to be compensated for from the input terminal 1 is destroyed. As a result, the lost low frequency component in the digital signal is not fully compensated for, so that the possibility is increased that an error occurs in determinating data.
The following measures to solve such a problem, are considered: (1) to increase the smoothing time constant of the level detector 10, and (2) to obtain an input to the level detector 10 from an output terminal 4 of the adder 2. However, in the above described method (1), a control signal is delayed as the smoothing time constant is increased, for example. In addition, in the method (2), a feedback ratio must be restrained low so as to prevent oscillation of a system, so that compensation of a low frequency component is not fully made.
Meanwhile, as a supply source to the DC restoration circuit of the above described digital signal having its low frequency component lost, a recording/reproducing apparatus of the digital signal is known as described above. A typical recording/reproducing apparatus is a digital VTR for recording/reproducing a PCM (Pulse-Code Modulation) video signal utilizing a plurality of rotary heads.
FIG. 7 is a block diagram showing a reproducing system in such a conventional digital VTR, and FIG. 8 is a waveform diagram for explaining an operation of the digital VTR shown in FIG. 7.
In FIG. 7, recording/reproducing heads 13A and 13B are provided opposed to each other by 180.degree. on a guide cylinder 11. In addition, a magnetic tape 12 serving as a recording media is wound around the guide cylinder 11. Windings 14A and 14A' and 14B and 14B' respectively constitute rotary transformers, which are connected to pre-amplifiers 15A and 15B respectively provided corresponding to heads 13A and 13B. Outputs from the pre-amplifiers 15A and 15B shown in FIGS. 8(a) and 8(b), i.e., reproduced outputs of the heads 13A and 13B are alternately selected by a switch 18. A PG head 22 for detecting rotation of a cylinder detects magnetic flux of each of magnets 21A and 21B provided on the guide cylinder 11. The magnets 21A and 21B are reversed in polarity. Thus, a signal having its polarity alternately reversed as shown in FIG. 8(c) is obtained from the PG head 22. An output signal of this PG head 22 is amplified by a amplifier 23 and then, separated for each polarity by comparators 24A and 24B. Outputs of the comparators 24A and 24B are respectively delayed by a predetermined time by delay circuits 25A and 25B, to be applied to a set input and a reset input of an R-S flip-flop 26. More specifically, the R-S flip-flop 26 is alternately set and reset by outputs of the delay circuits 25A and 25B. An output of the R-S flip-flop 26 shown in FIG. 8(d) is used for switching control of the switch 18 as an RF switching pulse. For example, a terminal 16A is selected and connected to a terminal 17 when the output of the R-S flip-flop 26 is at an "H" level, while a terminal 16B is selected and connected to the terminal 17 when the output is at an 37 L" level. By such a switching operation of the switch 18, the outputs of the pre-amplifiers 15A and 15B come to be continuous as shown in FIG. 8(e ), i.e., combined, to be applied to a waveform equalizer 19. A reproduced digital signal from the wave form equalizer 19 is output through an output terminal 20.
Meanwhile, in the digital VTR shown in FIG. 7, a signal recorded on a magnetic tape 12 serving as a recording media comprises an effective signal section and a preamble portion and a postamble portion before and after the section, as shown in FIGS. 8(a) and 8(b). These preamble portions and postamble portion comprise a signal having a Nyquist frequency of data in the effective signal section, i.e., a signal having a frequency corresponding to the highest frequency of the recorded signal, in which portions there inherently exists no low frequency component unlike a data signal. A signal (in FIG. 8(e)) reproduced from the magnetic tape 12 by the reproducing system shown in FIG. 7 and output from the output terminal 20 is applied to a PLL (Phase-Locked Loop) circuit (not shown) after a DC component thereof is restored by the DC restoration circuit shown in FIG. 3 or 5. This PLL circuit causes a capturing operation to be completed during periods of the above described preamble portion and postamble portion in the input signal, to generate a clock signal synchronized with the signal having the above described Nyquist frequency. Determination whether data is at a high level or a low level in the effective signal section is made based on this clock signal. More specifically, as described above, if the preamble portion and the postamble portion are provided before and after the effective signal section so that the capturing operation of the PLL circuit is completed during the period thereof, a data determining operation in the effective signal section can be quickly performed.
However, in the digital VTR using rotary heads as shown in FIG. 7, intermittent signals as shown in FIGS. 8(a) and 8(b) must be made continuous by switching the switch 18, to be combined into an output signal as shown in FIG. 8(e). At the time point of switching of this switch 18, waveform distortion may be caused in the combined signal. As causes of such waveform distortion, the following are considered: (1) the difference in amplitute between reproduced outputs based on the respective differences in characteristics between magnetic heads and between respective transmission systems thereof, (2) discontinuity of phases of signals in the preamble portion and the postamble portion at the time of combination, and (3) spike-shaped switching noises in the preamble portion and the postamble portion.
FIGS. 9A and 9B are waveform diagrams for explaining a state in which such waveform distortion is caused. In FIGS. 9A and 9B, (a) shows a signal having a Nyquist frequency in the preamble portion and the postamble portion in the reproduced signal shown in FIG. 8(e) in an enlarged manner, and (b) shows a DC component extracted from the output signal of the reproducing system by the LPF 6 in the DC restoration circuit. In addition, in FIGS. 9A and 9B, a broken line represents timings for switching of the switch 18. For FIG. 9A, a signal cycle is continuous at the point in time of switching, so that an unnecessary low frequency component is not generated in an output of the LPF 6. However, for FIG. 9B, when reproduced signals from the heads are combined, a signal cycle becomes discontinuous at the point in time of switching due to the relative time difference. In this case, this discontinuous portion is extracted by the LPF 6 as a low frequency component, to be enlarged by the action of a feedback loop. The signal having a Nyquist frequency in the preamble portion is significantly lost due to such a low frequency component. Thus, the clock signal is not reproduced by the PLL circuit, so that it may be impossible to determine digital data. Furthermore, in order to prevent such a situation, it is necessary to set a longer period for the preamble portion. Consequently, the effective signal section must be shortened, so that a considerable amount of data can not be recorded. On the other hand, the measures to prevent a malfunction caused by drop out of a input signal is disclosed in, for example, Japanese Patent Laying-Open Gazette Nos. 16275/1987 and 129975/1985.
Additionally, in the reproducing system in the digital VTR shown in FIG. 7, the rotary transformers 14A and 14A' and 14B and 14B' are respectively provided corresponding to the heads 13A and 13B. Thus, there exist differences in frequency characteristics, amplitude level and the like between the respective output signals (in FIGS. 8(a) and 8(b)) (such a difference is referred to as a characteristic difference between channels hereinafter). Thus, in the DC restoration circuit shown in FIGS. 3 or 5, if a constant, i.e, feedback loop characteristics of the DC restoration circuit are set corresponding to characteristics of one channel, i.e., a system corresponding to any one of the magnetic heads, a compensation error occurs with respect to the other channel, so that the possibility becomes large that an error occurs in discriminating date.